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Boulder storm summit: Questions linger among experts

Scientists gather at CU to discuss early research on devastating flood

By Charlie Brennan Camera Staff Writer

Posted:
09/25/2013 06:44:48 PM MDT

Updated:
09/25/2013 07:54:59 PM MDT

Researchers Klaus Wolter, left, a Cooperative Institute for Research in Environmental Sciences research scientist, and Martin Hoerling, a research meteorologist at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory, watch a projection screen during the talk called "Severe Flooding on the Front Range" at the University of Colorado on Wednesday.
(
Paul Aiken
)

The experts say it's still far too soon to draw many concrete conclusions from the record-breaking flood of 2013. Nevertheless, a number of scientists gathered at the University of Colorado on Wednesday for what sounded like an attempt to do just that.

"This is not intended by any means to be comprehensive, definitive or final," said moderator Jeff Lukas, a senior research associate at Western Water Assessment, a program of the Cooperative Institute for Research in Environmental Sciences at CU.

He joked during the 90-minute session that a four-page informational handout discussing the flood was merely "a caveat," representing a "compromise between rapidity and inclusiveness."

On a question that has sparked much discussion since Boulder saw a 17.15- inch deluge from Sept. 9 to Sept. 16 -- including just over 9 inches at its peak in one 24-hour period -- Lukas said a determination of the flood's rarity would differ based on location.

"It's likely this was a 100-year flood in a couple of drainages, including the Big Thompson and the St. Vrain" rivers. "Probably not in Boulder Creek, at least at the Broadway gauge, (where it was) something less than a 100-year flood."

But Lukas did not dispute the finding by the U.S. Geological Survey, announced while the storm was still raging and based on a 4,500 cubic feet per second flow in Boulder Creek to the east at North 75th Street -- that the flood represented at that location an event carrying a one-in-100 probability of occurring in any given year. The USGS has never retracted that statement but is also continuing its own assessments.

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"They are the acknowledged experts," Lukas said. "The determination of the magnitude of the flood and also estimations of the event probability -- that is certainly their expertise."

He said that although the rain was historically heavy in the city of Boulder, the rainfall totals were generally lower directly to the west -- for example, Lukas said "only" 7 inches fell at his home in Rollinsville. But in the mountains feeding the St. Vrain drainage, the rain was far heavier. Lukas cited one credible report of 17 inches in the tiny community of Riverside, close to Allenspark.

With much more extensive analysis to come in the months and likely even years to come, the recent storm, for its timing, duration and scope, is thought to be most analogous for the Boulder area to a storm that occurred in the first week of September 1938.

In that event, more than 6 inches of rain fell west of Eldorado Springs, with more than 80 percent of the total precipitation being dumped in the South Boulder Creek basin in the afternoon and evening of Sept. 2. That triggered a highly destructive flood, with a peak discharge of 7,390 cfs recorded at Eldorado Springs late that night.

'Shades of gray' in the data

Russ Schumacher, an associate professor in atmospheric science at Colorado State University, analyzed the 2013 storm through the lens of the National Oceanic and Atmospheric Administration's Atlas 14 precipitation frequency estimates. He concluded that Boulder's rainfall -- distinct from the flooding -- was actually a one-in-1,000-year event.

But Klaus Wolter, a CIRES research scientist who works in NOAA's Earth System Research Laboratory -- and lives just three miles above Jamestown, which was devastated by the flood -- took issue with that.

"I know the person who is responsible for these NOAA atlases, and I have actually argued with him about this," Wolter said. "I think they have been too conservative in how they estimate these return frequencies, and I think they all should read 'Black Swan.' They need to go back and study, how do you model extreme events?"

"The Black Swan," a 2007 non-fiction bestseller by Nassim Nicholas Taleb, explored the role of random events of great consequence on society and the ability to allow for, and withstand, them.

Nolan Doesken, state climatologist at CSU, made the point that it's risky to assign weather events their place in history too quickly because there are subtleties and "shades of gray" in the data that take time to be fully resolved. As an example, he said the respected paleoclimatologist Bob Jarrett, now retired from the USGS, concluded only long after the fact that the lethal Big Thompson flood of 1976 was -- in some areas of the canyon -- a one-in-10,000-year event.

"What gets published the first day gets remembered forever," Doesken said. "And they are always preliminary. ... And there's nothing to say you can't have multiple 10,000-year events."

Wednesday's session also promised to address the question of to what degree human-caused climate change might have contributed to the Front Range's recent storm.

As with the question of whether Boulder just witnessed an event of greater or less than 100-year rarity, the consensus appeared to be that the jury remains out on this question, too. There is no dispute, however, that warmer air contains more moisture, and meteorologists had taken note of a higher-than-average moisture content in the atmosphere in the unseasonably hot days just before the rain's onset the afternoon of Sept. 9.

"The most plausible influence of climate change that we can see (in the recent storm) is in making slightly more water vapor available for precipitation," Lukas said. Climate change "may have been responsible for a relatively small increase in the water vapor in the moisture plumes that fed the Front Range event."

'Nobody would have believed me'

Also on the table Wednesday was a discussion of how well forecasters did in predicting the rain the Front Range experienced. Here, too, there was no one hard, fast answer.

"There were alarm bells. It did look like a very significant event unfolding, I would say, at least 24 hours in advance," said Marty Hoerling, a research meteorologist with NOAA's Earth System Research Laboratory.

But Kelly Mahoney, a CIRES research scientist who also works in NOAA's Earth System Research Laboratory and is an expert on Front Range thunderstorms, offered a different perspective.

"As the event got closer, things started to go off the rails. The picture (shown in forecast models) got a lot less clear," she said.

"The human aspect is going to get looked at very closely in the next few weeks, as the weather service puts together an assessment team to look at that. ... None of them did fantastic, consistently, across the board. It (what forecast models were showing) was very much a mixed bag."

In addition to the climatologists in the room, the session also attracted a weatherman more familiar to a wider audience: Mike Nelson, veteran forecaster at Denver's 7News.

On the tail end of the forecast discussion, Nelson said, "If I had gone on the air and said Boulder is going to get nearly a year's worth of moisture in five days, nobody would have believed me.

"And, frankly, there is enough trouble with people believing us anyway."

Nolan Doesken, state climatologist at Colorado State University, left, makes a point about the recent weather during a talk Wednesday about the historic flood. At right is CIRES researcher Kelly Mahoney.
(
Paul Aiken
)

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